In what way is he manipulative.
If you do not feel joint mediation will be of benefit you do not have to go.
Both parties need to show they have gone to an initial meeting individually, but there are many situations in which joint mediation isn't recommended.
After my stbx went to his initial meeting they wrote and told me they didn't think joint mediation would work for us. Wasn't going to go with him anyway (EA), so this just validated it.
My advice would be to write down what YOUR goals of the meeting are. Focus just on the children and their needs. The mediator should guide things back to the issue in question.
In your situation I would tell myself that saying nothing is better than trying to get embroiled in heady discussions, especially if he's being manipulative.
Also, it might sound a bit woo woo, but try a mantra you can just repeat in your head - something like "it's about the kids, it's about the kids".
Good luck and to you too duffel